This invention relates to stored program process control systems which perform in real time.
One example of a stored program process controlled machine which operates in real time is the No. 1 Electronic Switching System that is described in the September 1964 issue of the Bell System Technical Journal. Certain aspects of the No. 1 ESS including "Detailed Telephone and Maintenance Functions" are described in U.S. Pat. No. 3,570,008 which issued on Mar. 9. 1971. As described in the above-noted patent, the processor performs its operations at a number of levels of program interrupt and at a base level which comprises a plurality of sublevels A through E which are executed in an ordered sequence. This organization of interrupt levels and the base level, including the sublevel operations, is calculated to assure that the operations which must be performed in real time or near real time are handled reliably and that all system work including maintenance is routinely performed. It is desirable to fully utilize a process control system, however, such systems have physical limitations which define the load which a system can handle in real time or near real time. In a telephone switching system the calls presented to the system comprise the "load" and an automatic measurement of the "occupancy" of the processor of an illustrative switching system is disclosed in B. J. Eckhart and E. S. Hoover Pat. No. 3,623,007 which issued Nov. 23, 1971. In that patent, a measure of processor occupancy is the so-called E-to-E base level return time. The sublevels or classes of work which are performed at the base level in that processor are arbitrarily termed A through E, and E is the sublevel which is performed least frequently (once per defined period of time) but which must be performed regularly.
Work operations of a stored program process controlled machine can be divided into two broad categories, namely, "overhead work" which is cyclically performed and which must be performed whether or not any "load" is presented to the machine and "load related work." Load related work may occur at a number of interrupt levels and at base level. Typically, near real time input functions, e.g., scanning of subscribers lines to detect request for service are performed in an interrupt level and detected requests are identified and stored in "hoppers" for subsequent processing at base level. The act of identifying requesting lines and storing that information in the hopper is an example of "load related work" which is performed at an interrupt level. The subsequent processing of the requests which have been placed in the "hoppers" and served at base level is an example of "load related work" performed at base level.
In adapting a large program controlled machine to perform new tasks or to perform old tasks in a more efficient manner, a considerable amount of programming, or reprogramming effort, is typically required. A new program may entail hundreds of man hours of effort and involve the labors of many people. Such a program should not be released to the field until it is sufficiently tested to assure its reliability. Thorough testing would involve running the program at different levels of offered traffic including high levels of processor occupancy. As can be appreciated from a consideration of the above-mentioned Eckhart-Hoover system, the central processor will traverse certain program loops only under very heavy traffic conditions. Heretofore, it has been very difficult effectively to load down large and powerful processors because to do so requires the artificial generation of heavy traffic.
Whether the traffic is simulated through signals applied to the peripheral buses or is simulated by load devices attached to the telephone network and programmed to generate various types of traffic in a random fashion, massive amounts of circuitry are required to provide a range of loads.